Several methods and apparatus are available for the analysis of fluid composition based on investigation of the acoustic impedence of the fluid which changes with various fluid properties. These devices in general use either the measurement of the transit time through the fluid of pulses of acoustic energy or the measurement of the phase shift of a constant frequency acoustic wave energy signal applied to the fluid.
Using a fixed distance between the transducer for imparting the signal to the fluid and the transducer for receiving the signal after it transits the fluid simplifies instrumentation and reduces the possibility of error. With a single frequency system, however, a fixed transducer separation causes problems because of the effects of constructive and destructive interference (standing waves).